Correlation between assessment of cytochrome P450 1A2 activity and enzyme activity scores, and their relation to clozapine exposure.

Aims: Cytochrome P450 1A2 (CYP1A2) is involved in the metabolism of antipsychotic drugs such as clozapine and olanzapine. Personalization of these treatments requires an accurate estimation of CYP1A2 activity. In this study, we aimed (1) to evaluate the correlation between activity score (AS), covariate-corrected activity score (CCS) and the phenotype of CYP1A2 using a caffeine test probe and (2) to investigate their relationship with dose-adjusted clozapine concentrations in a subgroup of the cohort.

Methods: A multicentric, retrospective and observational study was carried out in the French university hospitals of Marseille and Tours. CYP1A2 activity was calculated by the paraxanthine/caffeine (17X/137X) ratio determined 4 h after an oral intake of 100 mg caffeine. AS was calculated according to the CYP1A2*1F alleles. CCS was calculated according to the CYP1A2*1F alleles, smoking status and the presence of concomitant inhibitors.

Results: As expected, among the 89 patients included, the 17X/137X ratio was significantly higher in patients who smoked. We found a significant but modest correlation between the 17X/137X ratio and CCS (R  = 0.3, P = 1.74 × 10 ) but none between the 17X/137X ratio and AS (R  = -0.007, P = 0.52). AS was not correlated with dose-adjusted clozapine levels, contrary to CCS (R  = 0.19, P = 0.016) and especially the 17X/137X ratio (R  = 0.42, P = 1.7 × 10 ).

Conclusions: Correlation with clozapine concentrations showed the advantage of the 17X/137X ratio over the CCS in clozapine dose optimization. CYP1A2 activity, especially when determined by the caffeine probe, may be used to personalize clozapine dosing for patients experiencing treatment failure.